The present invention relates in general to video scrambling techniques for the protection of television transmissions, and more particularly, to a scheme for digitally-controlled analog encryption of video signals to be transmitted via satellite communication.
A number of schemes for scrambling television signals have been proposed in recent years, particularly with the advent of subscription television wherein subscribers pay to receive noncommercial television programs generally transmitted to their receivers via coaxial cables. In such systems, the secure transmission of television programs to authorized subscribers only is essential to the commercial success of the system, and therefore, encoding systems of varying complexity have been developed to prevent unauthorized receipt of such programs by those who have not paid for this service.
One general example of a system for scrambling and unscrambling communication signals is disclosed in the Hartung et al. U.S. Pat. No. 3,919,642, issued on Nov. 11, 1975, in which video scrambling and unscrambling are effected by inversion of selected horizontal lines of a transmitted television picture in accordance with various possible modes including, inversion of alternate groups of equal numbers of lines, inversion or noninversion of lines selected on a line-by-line basis in accordance with a control signal transmitted with each line, and inversion or noninversion of lines in a preselected sequence in accordance with the output of a suitable random noise generator. Along with the scrambling and unscrambling of the video signal, this prior art system also provides for scrambling of the audio portion of the transmitted signal by offsetting the audio carrier and thereby increasing the frequency difference between the video and audio carriers.
Another such system is disclosed in the Harna et al. U.S. Pat. No. 3,732,355, issued May 8, 1973, in which a video signal is encoded by switching it alternately between delayed and undelayed modes several times during each field in response to a locally-generated phase-varying rectangular switching signal which is varied randomly at random intervals in response to a series of random-state control pulses from an inhibitable random pulse generator, giving a jittered effect to the picture as the alternately-displaced bands vertically shift position in a random manner. The inhibitable random pulse generator generates an output pulse indicative of a random one of a series of possible counting states, one or more of which may be inhibited to narrow selection of the remaining states. A counter included in the generator first has random noise applied to it for a predetermined period of time, leaving the counter in an unpredictable random counting state, and then, stepping pulses are applied to step the counter to the first uninhibited state, at which time counting action ceases and a representative output pulse is developed to control the switching between modes.
Another system which is based upon the inverting of randomly-selected fields to effect scrambling of a television signal is disclosed in the Horowitz U.S. Pat. No. 4,075,660, issued Feb. 21, 1978. In this system, the transmitted signal is encoded by reversing the polarity of the video signal during randomly-selected fields and encoding bursts are injected into the composite signal prior to transmission to indicate whether or not the subsequent field has a video portion to be inverted. For purposes of encoding the audio portion of the transmitted signal, the program audio signals are modulated onto a suppressed carrier which is derived from the horizontal synchronizing signals and has a frequency equal to twice the horizontal line frequency.
In addition to systems which utilize synchronized noise generators at both the sending and receiving locations to effect encoding of the transmissed television signals, various coding systems such as disclosed in the Sherman U.S. Pat. No. 4,081,832, issued Mar. 28, 1978, and the Loshin U.S. Pat. No. 4,025,948, issued May 24, 1977, have been provided wherein the program to be broadcast is encoded to effect electrical inversion of selected groups of lines of the program thereby to scramble and disguise both the video and audio signals, and simultaneously with the coding of the program, a punchcard is prepared which is forwarded to the subscriber for use in enabling a decoder associated with the subscriber television set to permit receipt of the transmitted television signals.
In addition to the fact that the foregoing systems are often quite complicated and expensive, sometimes involving the use of punchcards for decoding, the various schemes for scrambling the video and audio signals have often proven to be less than completely foolproof. However, even more importantly, while these prior art systems may operate in a satisfactory manner in the specific environment for which they have been particularly designed, that is, transmission directly to the subscriber via coaxial cable, various problems arise when such systems are called upon to operate under different conditions and circumstances. For example, where the video communication is to be by way of satellite communication, it is extremely important to maintain a very high signal quality since the video signal must be retransmitted for distribution. Thus, if the particular encoding scheme which is adopted results in an inherent deterioration of the signal quality, the system becomes impractical for such communication.
One of the particular problems which results from the selective inversion of horizontal lines or combinations of horizontal lines in successive television fields, as suggested in the above-mentioned prior art systems, relates to the introduction of differential nonlinearities in the transmitted signals due to gain variations and level shifting in the system with the result that the dynamic range and quality of the transmitted television picture often suffers dramatically, resulting in a "washed out" picture. In particular, with the inversion of video information, a loss in the black level reference typically results which directly affects the picture quality of the transmitted video signals. Efforts to restore the black level of the video signal upon unscrambling by clamping the signal levels at the receiver to predetermined values have proven to be less than completely satisfactory, since such compensation fails to provide a dynamic correction of the problem which requires correction of both throughput gain and throughput offset.